CA2194349C - Method and apparatus for extracting precious metals from their ores and the product thereof - Google Patents

Abstract

A method and apparatus (4) for extraction of precious metals from their ores and the product (26) thereof. Oxidized ore comprising precious metals is exposed to a leaching solution (lixiviant) (254) comprising a relatively high concentration (activity) of dissolved hydrogen sulfide gas, a relatively high concentration (activity) of bisulfide ions, and a relatively low concentration (fugacity) of dissolved hydrogen gas. The hydrogen sulfide gas and bisulfide ions are preferably added to the solution by sulfate-reducing bacteria growing in a medium comprising dissolved sulfate ions and dissolved nitrate ions, but abiotic sources may also be used. Examples of such bacteria include mesophilic, fresh-water species such as Desulfobacterium catecholicum DSM 3882 and Desulfovibrio simplex DSM 4141; mesophilic, salt-water species such as Desulfovibrio salexigens DSM 2638; and thermophilic, fresh-water species such as Desulfomaculum kuznetsovii VKM B-1805.

Description

r ' w : 'y METHOD AND APPARATUS FOR EXTRACTING PRECIOUS METALS
FROM THEIR ORES AND THE PRODUCT THEREOF
STATEMENT AS TO RIGHTS IN INVENTIONS
MADE UNDER FEDERALLY-SPONSORED
RESEARCH AND DEVELOPMENT
This invention was made with U.S. Government support under Small Business Innovation Research Grant No. DI~tI-9461234 which was awarded by the National Science Foundation (NSF), an independent agency ofthe U.S. Government. The U.S. Government has certain tights in the invention.
TECHNICAL FIELD
This invention relates to a method and apparatus far extracting precious metals from their ores and the product thereof. In particular, it relates to the fbllowing: (1) a biohydrometallurgical process and apparatus for extraction and recovery of gold, silver and platinum group elements from their ores; (2} the products of that process and apparatus BACKGROUND ART

2 0 Development of cost-effective techniques far extraction of precious metals from their ores has been a goal of metallurgists for hundreds of years. In recent years, the addition of emzron-mental costs to the total cost of products of precious metal miners has encouraged a search for environmentally acceptable options, especially for refractory ores.
In the prior art the first step in precious metal production from ore involves preparing the 2 5 ore for precious metal extraMion. Preparation can take any one of a number of courses depending on the character of the are. Gold and silver ores ofren contain metallic sulfides. Ores containing platinum-group elements (PGE) typically also contain metallic sulfides. For example, in the Bushveld Igneous Complex in South Africa, platinum group element values occur in solid solution in the base-metal sulfides pyrrhotite (Fel_zS}, pentlandite (Fe,Ni)vSB, pyrite (FeS2), and as the 30 discrete platinaid metal minerals cooperate (PtS), faurite (RuS:), braggite (Pt,Pd,Ni)S and Pt-Fe alloys or their intergrowths (see Guilbert, J.1~4. 8. Park, C.F , Jr. "The aeolow~ of ore deposits."
New York: W.H. Freeman and Company, 1986}The 5tillwater Complex in Montana is a similar deposit Typically, refractory, non-oxidized (e.g., sulfide) Bald and silver ores (especially those with a relatively high carbon content) are oxidized at elevated temperatures and pressures in large autoclaves (i.e., "roasted"), prior to the extraction of precious raetals by means of cyanide leaching, (see McQuiston, Jr., F.W., & Shoemaker, R.S., Gold and Silver Cyanidation Plant Pr ctice, Vol. II, Baltimore: Port City Press, 1980).
During the Last decade, heap leach processes for cost-effective bio-oxidation of pyritic and arsenopyritic sulfides in gold and silver ores have been developed to the poim of commercial application (see Totma, A.fi., Biotechnologyr: A Comprehensive Treatise in 8 Volumes, Deerfield Beach, Florida: Verlag Chemie, I981). Recent improvements in the art are disclosed by: Pooley et al. in U.S. Patent No. 4,822,413, April 18, 1989; Hackl et a1. in U.S.
Patent No. 4,987,081, January 32, 1991; Hunter in U.S. Patent No. 5,076,927, December 31, 1991;
Brierly et al. In U.S.
Patent No. 5,127,942, July 7, 1992; and Brierly and Hill in U.S. Patent No.
5,246,486, September 21, 1993.
A great variety of precious metal extraction processes have also been developed (see Gupta, C.K., & Mukherjee, T.K., I~ydrometallurav in Extraction Processes, Vol. I, Boston:
CRC Press, 1990). Precious metal extraction processes are disclosed by: Pesic in U.S. Patent No. 4,778,519, October 18,.1988; Ball et al, in U.S. Patent No. 4,902,345, February 20, 1990;
and Kandemir in UK Patent No. 2, I 80,829, published April 8, 1987.
The relatively low economic cost ofcyanidation, however, has ensured its proliferation.
2 0 State-of the-art precious metal heap leach practice varies with the nature of the ore. Bio-oxidation process steps may include ore crushing, acid pretreatment, inoculation with appropriate sulfide-oxidizing bacteria, addition of nutrients, recirculating the biolixiviant and cooling the heap (for 3 to 8 days), and allowing the heap to "rest" (for 3 to 8 days). Precious metal extraction by means of cyanidation may include the process steps of washing the heap for an extended period (e.g., 14 days) to remove residual acidity or iron content, breaking the heap apart in order to agglomerate it with cement andlor lime to make a new heap, leaching it with an alkaline cyanide or thiosulfate solution for 30 to 40 days, and recovery of gold and silver from the leach solution by adsorption on activated carbon or zinc dust precipitation.
A significant amount of work in,the field of bio-oxidation and metals extraction has been accomplished by a variety of investigators. Tomizuka, N. & Yagisawa, M, in "Optimum conditions for leaching of uranium and oxidation of lead sulfide with thiobaci>yus ferrooxidans and recovery of metals from bacterial leaching solution with sulfated-reducing bacteria," (in et lu ' A Ii do o B a ' 1 Leac 'n d elat Mtcrob'o1 'cal om Murr, W0 96100308 , PCTIUS95I09199 ~' .; .' :, , L.E., Torma, A.E., & Brierly, J.A. (Eds.) New Yark: Academic Press, 1978), describe a two-step process for leaching of uranium and oxidation of lead sulfide where recovery of metals is accomplished by means of microbial sulfate reduction. Alper, J., in "Bacterial methods may strike it rich in refining metals, cleaning coal," (Hitfr Technolos;v. April, 1984, pp. 32-35), describes the bio-oxidation of gold-bearing arsenopyriteJpyrite and notes that production of large amounts of arsenic and sulfurous gases is avoided. Torma, A.E., in Biotechnoloev A
Comprehensive Treatise in 8 Volumes, (Deerfield Beach, Fl: Verlag Chemie, 1988), reviewed bioleaching processes. Livesay-Goldblatt, E., in Fundamental and Applied BiohvdrometallurQV, (Proc. 6th International Symposium on Biohydrometallurgy, Vancouver, B.C. 89-96, 1986), described a i G process for gold recovery from arsenopyrite!pyrite ore by bacterial leaching and cyarudation.
Torma, A.E., in "Biotechnala~~. A co>~rehensive treatise in 8 volumes,"
(Deerfield Beach, FL:
Verlag Chemie, 198x), reviews bio-oxidation of gold and silver ores. Hackl, R.P., Wright, F., &
Bruynesteyn, A., in Proceedines of the Third .Annual General Meetinc of Biominet, (August 20-21, 71-90, 1986), described development of the BIOTANKL.EACH process for leaching pyritic materials from gold and silver ore. The results of bench-scale and pilot-scale evaluations were presented. Marchant, P.B., & Lawrence, R.W., in "Flowsheet design, process control, and operating strategies in the bio-oxidation of refractory gold ores,"
(Proceedings of the Third Annual General Meeting ofBiominet, August 20-21, 39-51, 1986), listed considerations in the design of commercial bio-oxidation plants. Lawrence R.W , in "Biotreatment of Gold,"
2 0 (Microbial Alineral Recovery New York: MCGraw-Hill edited by Ehrlich, H.L.
and Brierly, C.L, 1990), discussed biotreatment of gold ore. The benefits of using the BacTech moderately thermophilic cultures in bio-oxidation processes were discussed by Budden.
J.R., & Spencer, P.A.
in "Tolerance to temperature and water quality for bacterial oxidation: The benefits of BacTech's moderately thermophilic culture," (FEA1S Microbiolos3y Reviews, I I, 191-196, 1993). Chapman, 2 5 J.T., Marchant, P.B., Lawrence, R.W., & Knopp, R., in "Biooxidation of a refractory gold bearing high arsenic sulphide concentrate: A pilot study;' (FEMS hlicrobiolo;y Reviews, I 1, 243-252, 1993), described a modular mobile bialeach pilot plant for bio-oxidation of a refractory gold-bearing high-arsenic sulfide concentrate. Moffat, A. S., in "Microbial mining boosts the environment," (Science 264, 778-779, 1994), disclosed bow bio-oxidation can increase the 30 efficiency of mining.
Thermophilic versus mesophilic biolea.ching process performance was evaluated by Duarte, 7.C., Estrada, P.C., Pereira, P.C., & Beaumont, F1.P. in " FEMS
Microbiology Reviews, 11, 97-102, 1993. Two years of BIO?~ bio-oxidation pilot plant data were analyzed by Hansford, WO 96l0030S 4 ~ , ~ ~ PCTlUS95t09199 G. S., & bliiler, D.&I. in "Biooxidation of a sold-bearing pyrite-arsenophyrite concentrate,"
FEMS Microbiolgs>v Reviews, 11, 175-182, 1993. Hoffman, W., ICatsikaros; N., &
Davs, G., in "Design of a reactor bioleach process for refractory gold treatment," {FEMS
Microbiology Reviews. 11, 221-230, 1994), described the design of a reactor bioleach process For refractory gold treatment Liu, X , Petersson, S., & Sandstrom, A., in "Evaluation of process variables in bench-scale bio-oxidation ofthe Olympias concentrate," (FEMS
Microbiolo~Reviews; I l, 207-214, 1993), presented an evaluation of the effects of process variables on pyritelarsenopyrite oxidation and gold extraction. Maturana, H., Lagos, Ii., Flores, V., Gaeta, ~1., Comeja, I'., ~
Wicrtz, J.1' , in "Integrated biological process for the treatment of a Chilean complex gold ore,'' {FEMS Microbiology Reviews, I l, 215-220, 1993), described an integrated biological process for treatment of a complex gold ore. Mineral sulfide oxidation by enrichment cultures of a novel thermoacidophilic bacteria were described by Norris, P.R. & Owen, J.P.
in"P4lineral sulphide oxidation by enrichment cultures of novel thecmoacidophilic bacteria," FEMS
Microbiolouv Reviews 11, >I-56, 1993). Rate controls on the bio-oxidation of heaps ofpyritic material I 5 imposed by bacterial upper temperature limits were described by Pantelis, G. ~ Ritchie, A.LM. in "Rate controls on the oxidation of heaps of pyritic material imposed by upper temperature limits an the bacterially catalysed process,' {FEMS Microbioloev Reviews, 1 I, 183-190, 1993). Bio-oxidation bacteria have been characterized in detail. Briefly, C.L., &
Brierly, J.A., in ".A
chemoautotrophic and thermophilic microorganism isolated from an acid hot spring," (Canadian T' Microhioloev, 19, 183-188, 1973), characterized a chemoautotrophic and thermophilic (70°C) microorganism isolated from an acid hot spring. De Rosa, M., Gambaeorta, A., &
Bullock, J.D., in "Extremely thermophilic acidophilic bacteria convergent wRh Sirlfolobus acrdacaldarzrrs," (J.
General Microbioloev, 86, I >6-164, 1975), characterized the extremely thermophilic (85°C), acidaphilic {pH I .0) bacteria Szrlfvlobus aridocaldarius.
2 5 While prior art has extensively studied and developed the bio-oxidation process for oxidizing metal sulfides present in gold and silver ore to expose or mobilize precious metal values, little attention has been given to biotechnologies for extracting (solubilizing) and recovering those values. Torma, A.E" in "Biotechnology: A Comprehensive Treatise in 8 Volumes,"
(Deerfield Beach, FL: Verlag Chemie, 1988), reviewed dissolution of gold by microorganisms.
Olson, G.J., in "Microbial oxidation ofgold ores and gold bioleaching," {FENIS
Microbialouv etters,119, I-6, 1994), reviewed microbial oxidation of gold ores arid gold bioleaehing. He described cyanogeruc microorganisms such as Chromo6acterirrnz violacerrm, and he noted that gold forms soluble sulfide and polysulfide complexes and suggested that biogenesis of partially-WU 9GI00308 t ~ ~ ~ PCT/I1S95109199 oxidized sulfur compounds may be a mechanism of gold dissolution. The U.S.
Bureau afMines, (office of Technology Transfer in U. S. Bureau of Mines Cooperative Research Opportunities, Washington, DC: U.S. Bureau ofMines, 1995), disclosed that the solubility ofgold in dilute polysulfide solutions at elevated temperatures and pressures and neutral pH
levels is comparable to the solubility of gold in cyanide solutions.
Investigators have hypothesized natural processes far gold solubilization involving specific sulfide-oxidizing bacteria such as Thiohcreillus ferroxidans and THiolxrcillus denitrifrcarrs (see Lyalikova, N.N. & Mokeicheva, L. Y. in "The role of bacteria in gold migration in deposits,"
Microbiolo~v, 38: 805-810, 1969; Kulibakin, V.G., Raslyakow, N.A., Tsimbalist, V G., Mel'nikova, R.D., and Nepeina, L.A., in "Role of sulfirr bacteria in supergene migration and concentration ofgold," Trans. Inst. Geol. Geofiz. Akad. SSSR Sib. Otd., 370:
75-86, 1977 (in Russian)) and sulfate-reducing bacteria such as De.sulfovibrio (see Meyers, W.B. "An hypothesis ofthe chemical environment ofthe Rand Goldfceld, South Afiica," U.S.
Geological Sun~ey Open-File Report 1389, 1970; Meyers W.B. "Precambrian pyritic gold-and uranium-bearing conglomerates," Geolos_zical Society of America, Abstracts with Programs, 3:
656-657, I971;
Myers, W.B.,"Genesis of Uranium and Gold-Bearing Precambrian Quartz-Pebble Conglomerates," Geological Satrev Professional Paper 1161-Arl, 1981; and Mossman, D.J. &
Dyer, B.D., "The geochemistry of Witwatersrand-type gold deposits and the possible influence of ancient prokaryotic communities on gold dissolution and precipitation,"
Precambrian Res. earth, 30: 303-319, 1985).
Speculation concerning the impact of biological organisms on the solubilization of gold and silver by bisulfide complexes is based on the findings of those who have studied the formation of precious metal-bisulfide complexes abiotically. Krauskopf, K.B., in "The solubility of gold,"
(Eceyomic Geoloev, 46, 858-870, 1951 ), disclosed that "gcdd may be transported in alkaline sulfide solutions, even in dilute solutions near the neutral paint" and "experimentally, one ofthe most perplexing facts about the chemistry of gold is its ability to dissolve in solutions of HS- of moderate concentration even at room temperature, whereas it dissolves in S'=
(i.e., more alkaline solutions) only in concentrated solutions at high temperature." Barnes, H.L.
in " ,~ochemistrv of hvdrothermal ore deposits." (New York: Hold, Rinehart & Winston, Inc., 1967), disclosed that 3 0 "gold is known to be soluble in alkaline solutions containing bisulfide ar sulfide ion" and that "when the pH is increased to 7.0 at constant ~;5, HS' increases relative to H2S and the solubilities (of silver) summarized by Anderson rise abruptly by a factor of 20." (See Anderson, G.M., "The solubility ofPbS in H,S-water solutions,'' Economic GeoIo~,v. 57, 809-829, 1962). Weissberg, S

WO 96100308 ~ ~ ~ ~ ~ ~ ~ 1'CTIUS95709199 B.G., in "Solubility of sold in hydrothermal alkaline sulfide solutions,"
(Economic Geolostv, 65, 551-556, 1970), disclosed that "in less alkaline solutions, where the HS' ion predominates, the experimentally determined solubility of gold ranges from 100 to 200 ppm Au in solutions containing from 0.2 to 0.3 moles NaHS/ICg solution at temperatures between I
50 and 250"C...."
and that "the present results are in good agreement with results given by Ggrvzlo and by Lindner and Gruner and substantiate the high solubility of gold in near neutral pH
bisulfide solutions."
(See Ogryzlo, S.P., "Flydrothermal experiments with gold," Economic Geology, 30, 400-424, 1935; and Linden l.L & Gruner, Lid'., "Action of alkali sulphide solutions on minerals at elevated temperatures," Economic Geology, 34, 537-560, 1939). Seward, T.hL, in "Thio complexes of gold and the transport of gold in hydrothermal ore solution,"
(Cxeochimica et cosmochimica Acta, 37, 379-399, 1973), disclosed that "...an increase in the bisulfide ion concentration at constant pH (or HSYH=S ratio) leads to higher gold soiubilities" and that "considerable quantities of gold may be transported in hydrothermal ore solutions as thio complexes, particularly in the near neutral pH region where the Au(HS)~° complex predominates."
Seward, T.M., in "The stability of chloride complexes of silver in hydrothermal solutions up to 350°C," (Geochimica et Cosmochimica Acts, 40, 1329-1341, 1976), disclosed that "the solubility data {up to 180°C) ofMelenfyev et al, suggest that Ag(HS); will probably be important in near neutral hydrothermal solutions...." {See hlelent'yev, B.N., Ivanenko, V.V., and Pamfilova, L.A.
"Solubility of some ore-forming sulfides under hydrothermai conditions,"
Rastvorimost' nekotorykh tudoobrazuyushkikh sul'fidov v g~drotermal' nvkh uslovivakh' Moskva, 27-102, 1968). Mountain, B.W. & Wood, S,A., in "Chemical controls on the solubility, transport, and depostion of platinum and palladium in hydrothermal solutions: A thermodynamic approach,"
(~onomic Geology, 83, 492-510, 1988), disclosed that "Westland states that Pt is soluble in alkaline sulfide solutions, possibly a [Pt{HS),(OH)~~j- complexes. Recent experiments ... have 2 5 yielded Pt concentrations of about 1 ppm after one month for Pt metal in contact with a 1.0 m Na_5 solution." (See Westland, A.D., "Inorganic chemistry ofthe platinum-group elements,"
Canadian Inst Mining Metallurgyr_Snec L'ol. 23, 7-18). Gammons, C.H. & Barnes, FLL., in "The solubilitg~ of Ag=S in near-neutral aqueous sulfide solutions at 25 to 300°C," (Geochimica et Cosmochimica Acta, 53, 279-290, 1989), disclosed that "Ag(HS): is the dominant silver species 3 0 in hvdrothermal fluids with near-neutral to alkaline pH, relatively low oxidation state, high total sulfide, and T - 300°C'...." Shenberger, D,M. & Barnes, H.L., in °Solubility of gold in aqueous sulfide solutions from 150 to 350°C," {Geochimica et Cosmochimica Acta.
53, 269-278, 1989), disclosed that "the fact that gold is soluble in alkaline sulfide solutions has been known since at least the 17th century..." and that "the high stability of Au(HS~' indicates that geologically significant quantities of gold can be transported in typical hydrothermal solutions." Wood, S.A.
& Mountain, B.W., in "Thermodynamic constraints vn the solubility ofplatinum and paDadium in hydrothennal solutions: Reassessmert of hydroxide, bisulinde, and ammonia complexing,"
(E~,onomic Geoloav, 84, 2020-2028, 1989), disclosed "... Pt solubilities on the order of 10 to 100 ppb are attainable in bisulfide solutions at alkaline pH at 25 degrees C" and that "Pt and Pd bisulfide complexes show a strong similarity to Ag and Au bisulfide complexes, where the Au(HS)Z complex predominates over a wide range of conditions." Gammons, C.H., Bloom, M. S., do Yu, Y., in "Experimental imrestigation of the hydrothermal geochemistry of platinum and palladium: I. Solubility of platinum and palladium sulfide minerals in NaCIIH=SO, solutions at 300°C," (G~ochi~nc~ et Cosmochimica Acta, 56, 3881-3894, 1992), disclosed "... a broad similarity in the chemical behavior of Au and the PGE elements."
In evaluating the potential for transport of precious metals in natural systems as bisulfide complexes in hydrothermal fluids, investigators have assumed the hydrogen fugacity in their abiotic.systems is set by the mineral assemblages through which the fluids would move in nature.
For example, Weissberg, B.G., in "Solubility of gold in hydrothermal alkaline sulfide solutions."
(Economic GeoI~r, 65, 551-556, 1970), disclosed that "... the solubility of gall in natural systems depends on the hydrogen fugacity, which is conuolled principally by equilibria between the minerals pyrrhotite, pyrite, magnetite and hematite." Seward, T.M., in "Thin complexes of , gold and the transport ofgold in hydrothermal ore solutions," (Geochimica et Cosmochimica Acta, 37, 379-399, 1973), disclosed that "since the dissolution ofgold is a function of hydrogen fugacity (see, for example, Raymahashay, B.C. & Holland, H.D., "Redox reactions accompanying hydrothermal wall rock alteration," Economic Geolo~v, 64, 291-305, 1969), a pyrite-pyrrhotite 'redox' buffer was present in all experiments in order that the fm was maintained at a known 2 S value."
In the field of biological waste degradation, investigators have long understood and utilized the bioprocessing opportunities preserted by "interspecies hydrogen transfer" betroveen hydrogen-producing and hydrogen-consuming anaerobic microorganisms. A
biohydrometallurgical application of this knowledge was disclosed by Hunter, R.M., in "Biocata~rzed Pa ' Demineralization of Acidic Metal-Sulfate Solutions," (Ph.D.
Thesis, Montana State University, 1989) and by Hunter, R.M. in "BiocataI3rzed Partial Demineralization of Acidic Metal-Sulfate Solutions," (U.S. Patent No. 5,076,927, December 31, 1991), Microbial hydrogen :,- i ;
R'O 9GJ00308 ~ ~ ~ ~ ~ ~ ~ ~ ~ PCTlL1S95109199 management techniques are disclosed by Harper, S.R. & Pohland, F G. in'"Recent developments in hydrogen management during anaerobic biological wastewater treatment,'"
(Biotechnola and Bioen insT~g, 28, 585-602, 1986). The following reactions illustrate the consumption of hydrogen by acetogens (ACET), methanogens (METH), sulfate-reducing bacteria I;SRB) and nitrate-reducing bacteria that produce ammonium (.AMM) and nitrogen (NRB):
Acetogens (acetogenic bacteria):
2HC0; + 4H, + H* --> CH3C00' + 4H,0 aGn = -104.6 kJ
Methanogens (methanogenic bacteria):
HCO,~+4H_+II' -->CH,+3H,0 aGa =-135.6 kJ
Sulfate-reducing bacteria:
SOi a + 4H$ + H' --> HS' + 4H_O aGa = -1 S 1.9 kJ
Nitrate-reducing bacteria that produce ammonium:
NOJ + 4H=+ 2H'--% NH; + 3H;0 aG,; _ -599.6 kJ
Nitrate-reducing bacteria that produce nitrogen gas:
2N0,-+SHr+2H"-> tv=+6H=O aGa =-1,120.5 kJ
The negative free energies of these reactions at pH 7.0 (aGo ) indicate it is thermodynamically feasible for oxyanion-reducing, hydrogen-consuming bacteria to reduce hydrogen gas fiigacities in reactor environments to very low levels under anaerobic conditions.
Na single prior art reference or combination of references have suggested combining the 2 0 knowledge of the above lines of inquiry: the art of solubilization and transport of precious metals in hydrothermal fluids, and the arts of aerobic and anaerobic biopracessing.
The prior art does not teach the use of bio-oxidation to liberate (mobiliae) platinum-group elements from their ores.
The prior art does not teach the use ofsulfate-reducing bacteria to increase the fugacity of hydrogen sulfide gas and the activity of bisulfide ions in a reactor in order to increase the 2 5 solubility of precious metal-bisulfide complexes. Neither does the prior art teach the use of microorganisms capable of biological reduction of an oxyanion to lower the hydrogen gas Fugacity in a reactor in order to increase the solubility of precious metal-bisulfide complexes in the reactor.
In fact, the prior art teaches away from the present invention toward aerobic processes far -teaching of precious metals from their ores. Such aerobic processes are disclosed in the following 30 recently published books on the subject: Ehrlich, H.L. ( 1990), Microbial Mineral Recovery, New Fork: McGraw-Hilt; Gupta, C.K., & Mukherjee, T.K. (1990), Hvdrometallurgy in Extraction Processes. Vols. I and II, Boston: CRC Press; Yannopoulos, J.C.
(1991), The w0 96!00308 ~ ~ ~ ~ ~ ~ ~ PCf/US95109199 Extractive Metallur of Gold, New York: Van Nostrand Reinhold; Marsden, J. &
House, I.
( 1993), The Chemistry of Gold Extraction, New York: Ellis Horwood.
DISCLOSURE OF INVENTION
For the purposes of this disclosure, the term "ore" refers to a composition that comprises precious metal values. Thus, ore may be a mineral assemblage that is being mined in-situ (in place) or that has been mined conventionally; or it may be a waste product, such as obsolete or damaged electronic components. The term "precious metals" refers to gold(Au), silver(Ag) and/or platinum-group elements (PGE). The term "platinum-group elements"
refers to platinum I O (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Rh) and iridium (Ir). The term "bisufide lixiviant" refers to an aqueous solution comprising HS° ions, and may also comprise dissolved H,S gas (HzS~,q,). The term "bisulfide complex" refers to a complex comprising a precious metal and bisulfide.
The present invention provides method and apparatus for leaching of precious metals from 15 their ores by means of a leaching solution comprising a sulfide ion and having a low fugacity of hydrogen gas. Leaching is accomplished by formation of precious metal complexes. For example, in gold leaching at neutral pH's, the complex Au(HS)_ predominates.
At a pH above about 10, the solubility of gold is increased by the formation of the complex Au,S_°'-. Below a pH
of about 3, the solubility of gold is increased by the formation of the complex AuHS° Thus, 20 formation of a variety of precious metal-sulfide complexes is possible.
The invention may be practiced on oxidized ore, suflide ore, or otherwise refractory ore in a tank reactor or heap (each operation. Preferably, a bio-oxidation step for removing base-metal sulfides from precious metal ores is coupled with a bisulfide precious metal leaching step, but conventional roasting may also be used to remove base-metal sulfides and produce an acidic, 2 5 sulfate stream. Preferably, the leaching solution is essentially neutral or allcaline. In a preferred embodiment, the process ofproducing the leaching solution is biocatalyzed.
In a preferred embodiment, a first process step of bio-oxidation of are particles is accomplished to free (liberate) precious metals dispersed or occluded within the ore. A portion of the acidic, base-metal sulfate leach solution produced by the bio-oxidation step is introduced to an 30 anaerobic reactor. In a heap leach embodiment ofthe process, the anaerobic reactor is a side-stream reactor or a series of such reactors in series. In one alternative slurry (e.g., vat or tank) leaching embodiment, the anaerobic process may occur on-line. One or more preferably non-toxic electron donors (such as hydrogen gas, formate, acetate andJor methanol--which does not WO 96109)308 ; +° '~ ~ ~ ~ l(. ~ j~, ~ PCTlU895/09199 bind effectively to activated carbon) and growth requirements (such as vitamins and!'or salts), are added to the anaerobic reactor to enrich within it a culture of at least one oxyanion-reducing bacterium (e.g., a sulfate-reducing bacterium). In an alternative embodiment, the electron donors andlor growth requiremems are derived from organic material deposited on the ore by sulfide-s oxidizing bacteria during the bio-oxidation step. The hydrogen fugacity in the reactor, or at least in the last reactor in a series of such reactors, is maintained at a low level by at least one hydrogen-consuming bacterium. The anaerobic reactor may be operated in a pH-slat mode by adding sufricient acidic sulfate solution to maintain a neutral pH in the reactor (see Hunter, R.M., $i~ocatalvzed Partial Demineralization of Acidic Metal-Sulfate Solutions, Ph D. Thesis, Montana 1 c) State University, 1989). In an alternative embodiment, the anaerobic reactor may be operated in a sulfide-slat mode by adding sufficient sulfate solution to maintain a constant dissoh~ed sulfide concentration in the reactor in response to sistnals from a sulfide sensor (e.g., sulfide ion selective electrode), Aase metals are preferably precipitated and removed and a portion of the hydrogen sulfide gas (H=S) produced in the anaerobic reactor is preferably removed. In this way, oxyanian-i 5 reducing bacteria are used to create an essentially neutral leaching solution comprising a relatively high concentration bisulfide ions, a high fugacity of hydrogen sulfide gas, a low concentration of dissolved base metals and a low firgacity of hydrogen gas.
In an alternative embodiment, the precious metal leaching solution is produced in an anaerobic environment by contacting a stream of gas comprising hydrogen sulfide gas and 2 Q essentially no hydrogen gas with the solution until the environment has an appropriately' high concentration of hydrogen sulfide gas and an appropriately tow firgacity of hydrogen gas. Tlre gas may be produced biotically by a culture of sulfate-reducing; bacteria, or it may be produced abioticaliy by purifying HxS gas to remove Ha gas.
In a second process step, the oxidized ore (possibly in a heap that is covered and 2 5 submerged to exclude oxygen) is leached (by recirculating the neutral or alkaline bisulfide tixiviant comprising, or saturated with, H1S) in a leaching reactor. In one embodiment, the HrS partial pressure is increased by introducing the lixiviant under pressure at the bottom of a heap submerged in water, causing ion concentrations to increase in direct proportion to the increase in H=S partial pressure.
30 In a preferred embodiment, the anaerobic reactor and the leaching reactor are operated together as a single, essentially completely-mixed reactor. A completely mixed reactor is one that produces an ett7uent concentration of a conservative tracer (e g., a non-reactive dye) equal to 37 +3 percent ofthe initial tracer concentration (i.e., tracer mass divided by liquid volume) one WO96/00308 .. 1 ~':} ~ ~ ~ ~ ~ ~ Oj PCT~S951(19199 detention time (i.e., liquid volume divided by liquid volumetric flow rate) afrer an impulse input (i.e., slug addition) of the tracer The complexed precious metal (e.g., gold and silver) is recovered (preferably continuously) from the lixiv~iant solution. Recovery may be accomplished in a conventional manner by adsorption on activated carbon or by modifying either the solution pFL, hydrogen fugacity, or oxidation-reduction potential (ORP).
Recovered precious metals are converted into products. TMs may include the operations of separating, smelting and casting of each precious metal into bars, bullion or other forms.
The present invention offers a variety of advantages not provided by the prior art. One object of the invention is to lower the monetary cost of gold, silver and platinum-group element production. By utilizing a waste product (excess sulfuric acid from a roasting or bio-oxidation pretreatment step) as the starting material far preparation of a bisulflde lixiviant, the lixiviant (a neutral bisulfide solution) would be produced biologically instead of being purchased. Another object of the invention is to use bath inorganic (salts) and organic (biofllm carbonaceous compounds} byproducts of biaxidization as inputs to a precious-metal solubilization process.
Another object ofthe invention is to lower the environmental risk of precious metal mining. TMs is the case because the actual and perceived environmental risk of maintaining a large inventory of a neutral bisulflde solution is much lower than that associated with maintaining an equivalent volume of caustic cyanide solution. Another object of the invention is to provide a method and 2 0 apparatus for both in-situ or ex-situ (conventional) mining. Further objects and advantages of the invention will become apparent from consideration of the drawings and the ensuing description.
BRIEF DESCRIPTION OF DRAWINGS
The features of the invention will be batter understood by referring to the accompanying 2 5 drawings which illustrate presently preferred embodiments of the invention.
In the drawings:
Fig. 1 is a highly schematic block diagram illustrating a first representative embodiment of the present invention.
Fig. 2 is a Mghly schematic block diagram illustrating a second representative embodiment 3 0 of the present invention.
Fig. 3 is a Mghly schematic block diagram illustrating a third representative embodiment of the present invention.

WO 96100308 s. ; ~;i ~ ~ ~ ~ ~ ~ ~ PCTfU895109199 Fig. 4 is a highly schematic block diagram illustrating a fourth represemative embodiment of the present invention.

The following reference numerals are used to indicate the parts of the insrention on the drawings:

2 are 4 bio-oxidation reactor 6 sulfate ions 7 electron donor 8 sulfate reduction reactor 10 bisulfide lixiviant oxidized ore 22 bisulfide leaching reactor 24 pregnant solution 26 precious metals recovery reactor 15 28 leached ore ore 32 crushing 34 crushed are 36 acid leaching 20 37 aerobic reactor 38 air acid-leach solution 42 pump 44 acid-leached ore 25 46 bisulfide leaching 47 essentially completely-mixed, anaerobic reactor 48 bisulfide lixiviant pump pH controller 3 62 valve 64 valve 66 pregnant bisulfide lixiviant 68 gold and silver recovery W096100308~ (j ~ ~ ~ ~ PCTIUS95109199 70 spent Lixiviant 76 bisulfide lixiviant recirculation loop 78 activated carbon column 80 leached ore 82 sensarlcontroller 84 electron donor 90 dewatering 92 contained bisulfide lixiviant 94 waste ore 96 acid-leach solution portion 98 base metal removal 100 base metal removal reactor 102 acid leach solution portion 104 iron and other base metals I 10 excess hydrogen sulfide gas 112 excess hydrogen sulfide gas portion I 14 sulfur recovery 116 sulfur recovery reactor 120 elemental sulfur 2 200 heap 202 heap, second heap 204 crushed ore 205 crushed ore, oxidized ore 206 air 208 plenum -210 acidic"base-metal sulfate leach solution 212 pump 214 portion 216 distributor 3 220 portion 230 anaerobic, sulfate-reduction reactor 232 pH controller 234 valve WO 96!00308~ ! ~ ~ ,~ 4 ~j PCTlU595l09199 238 hisuifide leach solution 240 non-toxic eiectron donor 244 base metals 250 settling tank 252 portion 2S4 bisulfide lixiviant 260 headspace 262 headspace 264 conduit 266 excess hydrogen sulfide gas 270 sulfurrecovery 272 elemental sulfiu 282 bisulfide lixiviant 284 plenum 286 pump 290 distributor 292 portion 294 portion 300 pregnant portion 302 reactor 306 barren lixiviant solution 312 aerobic, stirred batch reactor 314 air pump 318 pH monitor 320 liquid supernatant 322 ore 324 portion of dried, bio-oxidized ore 326 continuously stirred tank reactor 328 media reservoir 330 pump 332 pump 334 H,S canister 3 36 pH monitor/controller W096100308 ' : PCT/US95/09199 19~..~~9 338 acidic supernatant 340 pump 342 efrluent storage container 344 pump 346 pump 348 160-ml serum bottles MODES) FOR CARRYING OUT TIgE INVENTION
Reference is now made to Fig. 1 which is a schematic block diagram illustrating a preferred embodiment of the invention, with the dashed lines representing possible variations in the process and apparatus. Ore 2 is the input to the process and, under certain conditions, may be the only input to the process. In a preferred embodiment, ore 2 is crushed and may be otherwise treated to optimize bio-oxidation. In bio-oxidation reactor 4, oxidation of metal sulfides is accomplished to free or mobilized precious metals dispersed or occluded within metallic sulfides in ore 2.
Bio-oxidation reactor 4 produces a sidestream comprising sulfate ions 6 and acidity. In some instances, the sidestream also comprises biofilm carbonaceous compounds.
In an alternative embodiment, bio-oxidation does not occur and sulfate ions 6 are an input to the process Sulfate ions 6 may be a component of a waste stream, such as acid none drainage, or by-product of ore 2 0 roasting.
In a preferred embodiment, electron donor 7 is added to sulfate reduction reactor 8 so that sulfate ions 6 are biologically reduced therein. In a preferred embodiment, sulfate reduction reactor 8 is operated at a mean cell residence time low enough to cause essentially-complete (99+
percent) utilization of electron donor 7. In a preferred embodiment, sulfate reduction reactor 8 is operated in a pH-stat mode so as to maintain an essentially Constant pH (~ 0.1 pH unit) in reactor 8 and in bisulfide lixiviant 10 that it produces.
Oxidized ore 20 is introduced to bisulfide leaching reactor 22. In reactor 22, precious metal values in oxidized ore 20 are dissolved and complexed by means of bisul6de lixiviant 10.
Pregnant solution 24 comprising precious metal values is introduced to precious metals recovery reactor 26 fur precious metals recovery in a conventional manner by adsorption on activated carbon; or by modifying either the solution pH, hydrogen fiagacity, or oxidation-reduction potential (ORP). A product (e.g., gold bullion) is formed from said precious metal by smelting and casting. In one embodiment, leached ore 28 is disposed ofin a conventional manner (e.g., IS

WO96100308 ~._ ' ; .. ~ ~ ,!~ ~ PCT/t1S95/09199 permanent storage) and need not be treated for removal of lixiviant. In a preferred embodiment, leached ore 28 is washed andlar dewatered to remove residual Iixiviant 10 prior to disposal.
Lixiviant 10 removed frora leached ore 28 is used to wet andJar neutralize the acidic pH of incoming oxidized ore 20 andlor it is returned to leaching reactor 22.
It is well known in the art that the composition of ores varies widely, requiring optimization of the leaching step based on ore compositions and other local conditions. For this reason, design and operation of reactors 8 and 26 are preferably optimized for precious metal dissolution and complex formation. Depending on ore composition, design and/or operation are varied to achieve the followinst conditions in the reactor environment:
1. Maximize dissolved bisulfide concentration and dissolved HZS~,st concentration at the precious metal surface;
2. Optimize pH;
3. Minimize hydrogen fugaciy at the precious metal surface;
4. Maximize pressure;

5. Maximize temperature.
For gold andlor silver leaching, information on the aqueous chemistry of gold and silver bisulfide complexes and other chemical species likely to be present in a bisulfide lixiviant is used (see Barnes, H.L. led.), C~~ehemistryfHvdrothermal Dre Deposits, 2nd ed., New York:
John Wiley & Sons, 1979). Published information on the aqueous chemistry of gold and silver bisulfide 2 0 complexes and other chemical species likely to be present in a bisulfide lixi4iant are used to produce a mathematical model of the salubilization step. The model incorporates the data presented in Tables 1 and Z. In the model, stability and equilibrium constants are used to predict the direction of a reversible chemical reaction under certain standard conditions and under other conditions. The standard conditions are 1.0 molar (h~ concentrations of dissolved reactants and 2 5 products and I .0 atmosphere (.4tm) pressure of gaseous reactants and products. The temperature is usually taken as 25°C (298°K), but stability and equilibrium constants are reported at other temperatures as well.

W096/00308 9 ~ , '' ~ ~ ,~ ~ ~ ~ ~ PC1YUS95l09199 Table 1. Equilibrium Constants of Metal Sulfides and Sulfide Complexes Metalt'reaction Temperature, Ionic strengthLog K
C

Cadmium CDs + H=Siq, -~ Cd(HS)x 25 1.0 -4.57 CDs + HzS~,~~ + HS-- Cd(HS); 25 l .0 -2.69 CDs + HZS~,~~ + 2HS~-~Cd(HS); 25 l.0 -0.33 =

Copper Cu=S + SHS~ + H* - 2Cu(HS)3 - 22 2.1 to 4.4 +2.020.26 Gold Au~,~ + HS- -~ AuS- + O. SH2~R~ 25 -5.6' Au~,~ + H~S - .AuHS + O.SHa~~ 25 - -11.1410.2 Au,a, + H:S~,y~ + HS' --~ Au(HS)~20 - -6.1 + O. SH~R~

2Au + H=S~,q~ + 2HS-~ Au~S(HS=)~175 0.50 -2.14 + H=~~

2Au + S=' ~ 2AuS' 25 -2.02 2Au + 2HS~ + 0.502 --~ 2AuS- 25 - +30.35 + H=O

2Au + 2HS- ~ Au2S + HiiRa + SR 25 - -11.25 Lead PbS + HAS"q, + HS -~ Pb(HS); 25 0 -5.62 f 0.2 PbS + H:Si,~~ ~ Pb(HS)_~,y, 25 0 -7.6' PbS + 2H~S"q~ -~ PbS(H=S)z~,9, 200 0 -4 88 Mercury HgS + 2H=S"~~ - HgS(HxS)uwr 20 1.0 -4.25"

HgS + H3S~ 20 1.0 -3.50 ~ + HS' --~ Hg(HS);

w HgS+2HS---r HgS(HS=)' 20 1.0 -3.51"

HgS + HS' + OH' - HgSi = + H,Of,~,25 0 0.31 Silver Ag"~ + HZS~,y~+ HS' -~ Ag(HS)= 25 - -2.72f0.10 + O,SH_~;~

Ag*+HSwAgHSi,q, 20 1.0 +(3.30 Ag* + 2HSw-r Ag(HS)= 20 1.0 +3,87 2Ag(HS), -~ Ag=S(HS),=+H,S~,~, 25 1.0 +3.2 rY0 96100308 ' , ~ ~',, ~ ~ ~ .~f ~ j~' ~ PCTlC~S95/09199 Table 1. Equilibrium Constants of Metal Sulfides and Sulf de Complexes (cont.
j Metafreaction Temperature, °C Ionic strength Lo K
Silver (coot. ) Ag~S + HAS -~ 2AgSH 20 1.U -15.78 Ag=S + HjS + 2HS- --~ 2Ag(SH): 20 l .0 -8.05 Zinc ZnS + 2HZS~,sy -1 ZnS{H2S)u,qy 80 - -2.24 ZnS + HsSt,~y + HS' ~ Zn(HS); 25 I .0 -3.0 t 0 4 ZnS + H,Sy-,~, + 2HS- ~ Zn(HS,)~' 25 1 0 -2 6 ' Staichiometry unproven.
b'fotal sulfide is O.SM, and pressure is 1,000 bar.
'Less certain d Solid phase is metacinnabar ' Pressure is 1 bar Table 2. Stability (Formation) Constants Log stability constant by Metal' Liguid Gold, !3, Silver, (3, Platinum 13, Palladium 13, Chloride (Ch) 9" 5.4' 13.99 11.54 Bromide (Bc ) I2" 7.1' 15.4 14.9 Nitrite {NO-~) - - 24.1 21.0 Iodide {I -) 19.6' - 29.6 24,9 Thiocyanate (SCN-) 17.1" - 33.6 25.6 Thiourea CS (NH,)~ 23.3° 13.10' - -Thiosulfate (SOS z) 28.7a 13.3' 43.7 35.0 Bisulfide (HS-) 30.1' 17.43' ,..51 ...41 Cyanide (CN-) 38.3' 18,7' ,..78 63 ' Far bivalent-ion complexes at 25°C
36 " Source: Hancock, R:D., Finkelstein, N.P., & Evers, A. (1977). A linear free-energy relation involving the formation constants of pailadium(Il) and platinum{II).
J.ino~.nucl.Chem. 39. 1031-1034.
Source' Kotz, J.C. & Purcell, K.F. (199I). Chemistry & Chemical Reactivity Philadelphia, P.A: Sounders Col3ege Publishing.
° Source: Marsden, J. & House, I. (1993). The chemisnv of bald extraGtipn. New Yark:
Ellis Hanvood.
' Source: Renders, PJ. & Seward, T.hl. (1989). The adsorption ofthia gald(Ij complexes by amorphous As7S3 and Sb,S, at 25 and 90°C. Geochimica et Cosmochimica Acta. 53. 255-26'7.

WO 96!00308 b ''. ' ~ ~ (j j~ ~ ~ i~ PCT/US95/09199 Equilibrium constants can be derived in a number of ways. The stability constant for a reaction is related to the standard free energy change the reaction as follows:
nG° _ -RT In K
Ln K = -eG°/RT
logK =-nG°/2.30*RT
where oG° = free energy change under standard conditions, kJ
R = gas law constant = 0.0083 l4 kJl(mol*°K) T = absolute temperature in decrees Kelvin (°K) 2.30 = In 10 The value of the product (2.3*RT) is 4.1840* 1.3636 K calireaction = 5.705 kJlreaction at 25° C
and 5.935 kJ/reaction at 37°C.
An opportunity for the solubilization ofprecious metal (e.g., gold, silver, platinum and palladium) is created by the complex-forming reactions:
Au~x>+ HaSc.v + HS'-_> Au(HS)=-+ 0.5 H=cap Ag,x~ + H=S~,q, + HS' --> Ag(HS)= + O.SH~~a~
pt"~+ 2H_S~ay~ + 2HS- __> pt(HS), z + Hya>
Pd~" + 2HzS~,q, + 2HS --> Pd(HS), _ + Hnai The equilibrium and stability constants for the gold and silver reactions have been determined experimentally and are dependent on the temperature at which they occur.
Shenberger, D.M. & Barnes, H.L., in "Solubility of gold in aqueous sulfide solutions from 150 to 350°C," (Geochimica et Cosmochimica Acta, 53, 269-278, 1989), have derived the following equation for the temperature dependence of the first reaction (between 150° to 350°C):
log Krumsy = -9.383 * 10'/T= + 2170.4iT - 2.28I6 where T = temperature in degrees Kelvin The following equation applies between 25°C and 150°C:
log Ka"n,su = 3.32 + (_2,420/T}
Similarly, Gammons, C.H. & Barnes, H.L., in "The solubility of Ag2S in near-neutral aqueous sulfide solutions at 25 to 300°C," (Geochimica et Cosmochimica Acta, 53, 279-290, 1989), have derived the following equation for the temperature dependence of the second reaction:
log Kaa~sk = 0.439 - 943/T
The equilibrium and stability constants for the platinum group element reactions can be estimated using the methods disclosed by Hancock, R.D., )!inkelstein, N.P., &
Evers, A., in "A
linear free-energry relation involving the formation constants of palladium (II) and platinum (II),"

WO 9b100308 ,' y ,,.: ,; ,,~J ~ ~ E~." (~ J ~ ~ PCT7US95/09199 r rrn,rnal of Inors~uvc and Nuclear Chemistry, 39, 1031-1034, 1977} and Mountain, B. W. &
Wood, S.A., in "Chemical controls on the solubility, transport, and deposition of platinum and palladium in hydrothermal solutions: A thermodynamic approach," {Economic Geolo~y, $3, 492-510, 1988}, They have demonstrated that, for metals in the group Au, Ag, Pt and Pd, plots of the logarithms of the stability constants of one metal versus another are linear for a variety of Ggands.
These equilibrium and stability constants may be used to determine fhe equilibrium molar concentration of each complex using the following equations:
[.4u(HS),~]=K,~su * [HaScw>] * [HS~If[H3i~I]°.
[~~$(HS)x ~ = K~q<ttsu * ~zS<.ar] * [HS~]~[Hnza~
[Pt(HS); x] = K~sn * ~xsan~]x * ~S ]xl[Hxcs~]
[Pd(HS).s x] = IiParns~s * [HxS<.a>]= * ~S ]1~[Hxcx~]
The above equations are used to estimate the hydrogen fugacity required to solubilize gold and silver at various temperatures and at two reactant (H,S + HS-}
concentrations. Those estimates far gold and silver are presented in Table 3. In preparing the estimates presented in Z 5 Table 3, the following activity and fugacity coefficients are used: Au (HS)3 = 0.7, Ag (HS). = 0.7, HxS = 0 97, HS' = 0.7, and H3 = 1Ø
Since anaerobic digesters must be operated at Hxu~ fugacities below about 10' atmosphere (atm), achievement of the firgacities indicated in Table 3 are feasible. This would produce gold and silver concentrations in a pregnant bisulfide lixiviant of 0.1 to I.0 mgft and 1.0 to 10 mgll, respectively, which are the same concentrations that occur in pregnant cyanide heap leach solutions during conventional cyanidation, Table 3. Hydrogen Gas Fugacities Required to Soiubilize Gold and Silver 2 5 Log of hydrogen gas (I-Ix~e~) fugacity, atm, required at indicated temperature, °C, 2o achieve indicated metal concentration.
Metal concentration f ligand concentration° 25 35 65 Gold, U. I mgR Au 1,400 mg/I Fi:S,,q~+HS- -3.84 -3.32 -1.92 2,700 mgll H=S~,s~+I-IS~ -2.63 -2.11 -0.71 Gold, 1.0 mgll Au 1,400 mg!( HZS~,v~+HS- -5.84 -5.32 -3.92 2 700 m~/1 H S +HS- -4 63 -4.11 -2.71 WO 96100308 c~ , ~ ~ t~ ~ ,~ tt ~ PCT/US95109199 Table 3 (Cont). Hydrogen Gas Fugacities Required to Solubilize Gold and Silver Log of hydrogen gas (H~~g,)fugacity, atm, required at indicated temperature, °C, to achieve indicated metal concentration Metal concentration / liuand concentration'25 35 65 Silver, 1.0 mg/1 Ag 1,400 mg/1 HzS~,q~+HS- -2.22 -2.00 -1.4b 2,700 mg/1 H=Sy+HS- -I .O1 -0.79 -0.25 Silver, 10 mg/1 Ag 1,400 mgll H=5~,~~+HS- -4.22 -4.00 -3.46 2,700 mgJl H=S~,~~+HS~ -3.01 -2.79 -2.25 ' Concentration of each reactant (HZS or HS-) is about half of the indicated total at pH 7Ø
In a preferred embodiment, bisulfide ions are generated biologically (by naturally-occurring sulfate-reducing bacteria) at very low cost using an acidic waste product (bio-oxidation heap leach effluent) as the sulfate source. For example, with formate ion as the electron donor, the following reaction occurs:
8HCOO-+ 2SOs Z + 3H' -~ 8HC0; + H,5 + HS' With acetate ion as the electron donor, the follwving reaction occurs:
CH,COO- + SO; - -> 2HC0; -~ HS
A mass balance on typical heap leach et3luents has shown that biological reduction of contained sulfate ions produces bisulfide and sulfide ions in excess of the concentration required for 2 0 essentially complete base metals precipitation (see Hunter thesis, 1989).
Thus, production of a high-concentration bisulfide Iixiviant is passible. Moreover, H=S gas can be recovered from spent Iixiviant andlor leached ore by reducing the H=S gas partial pressure in the gas mixture in contact with said spent lixiviant andlor leached ore using a vacuum pump. More complete H,S
gas recovery can be achieved by acidifying the spent lixiviant and/or leached ore to a pH below 7.0 and/or by increasing gasiliquid interfacial area, (e.g., by forming the liquid into droplets).
A variety of techniques may be used to macimize bisulfide concentrations in the lixiviant.
The solubility of hydrogen sulfides in water decreases with increasing temperature (from about 7,100 mgll at 0°C to 3,000 mgll at 30°C under a partial pressure of one atmosphere), (Environmental Protection Agency, Process Design n4anual for Sulfide Control in Sanitarv Sewera_ee Systems, EP.A 625ii-74-005,October 1974). In accordance with Henry's law, the W096100308 a .f ' y PCTlI3S95109199 saturation concentration of H=S in water is directly proportional to the partial pressure of the gas in the atmosphere in contact with the liquid. Removal of HZS increases the pH of the solution (by removing protons). Moreover, the proportion of bisulfide ion (relative to H=S) increases with pH over at least the range pH 5-9.
The optimal pH for the bisulfide lixiviant solution for precious metal recovery is the pH
that maximizes the solubility of target precious metal compounds and the stability of their complexes. For example, Krauskopf, K.B., in "The solubility of gold" Economic Geolosay, (46, 858-870, 1951 ), noted that "one of the most perplexing facts about the chemistry of gold is its ability to dissolve in solutions of HS- of moderate concentration even at room temperature, whereas it dissolves in Sn (i.e., more alkaline solutions) only in concentrated solutions at high temperature." Schwarzenbach, von G. & Widmer, M., in "Die loslichkeit von MetaIlsuIfiden,"
(Helvetica Chimica Acta, 49, 11 I-I23, 1966), found that the solubility of silver was greatest at pH 7 at a temperature of 20°C in the presence of excess sulfide in the form o.f HxS, HS-, and S''.
l~lelent'yev, B:N., ivanenka, V.V., and Pamfilova, L.A., in "Solubility ofsome ore-forming sulfides under hydrothermaI conditions," (Rastvorimosf nekotorvkh rudoobrazuvushkikh suffidov v gidrotermal' n~h usloyvakh' Moskva, 27-102, 1968), found that the solubility of Ag=S increases with pH in the range pH 4-8 in the temperature range 100-300°C. Sewsrd reported that for gold in solutions of reduced sulfur "a pronounced solubility maximum occurs in the region of pH about 7." ( Seward, T.M., "Thio complexes of gold and the transport of 2 0 gold in hydrotherntal are solutions," Geochimica et cosmochimica Acta. 37, 379-399, 1973).
Options for reducing hydrogea fugaciry include bioprocessing hydrogen-management techniques Hydrogen-consuming bacteria (hydrogenotrophs) include such anion-reducing bacteria as acetogens, methanogens, sulfate-reducing bacteria and denitrifying (nitrate-reducing) bacteria in natural ecosystems, these bacteria participate in "interspecies hydrogen transfer."
2 5 . Examples of acetogens include Acetobacterium u.~oocti (ATCC 29683, DSM
1030, or DSM
2396) and Clostridium aceticum (ATCC 35044 or DSM 1496). Examples of hydrogen-consuming methanogens are numerous and include the mesophiles Methcurobrevibacter rumiuaratium (ATCC 35063 or DSM 1093) and Methanorarcina barkeri (ATCC 29786 or DSM 805), and the thermophife aL9etlwraobarterium thermcxrutomophicum (ATCC
29096 or 3 0 DSM 1053). Examples of hydrogen-consuming sulfate-reducing bacteria are shown in Table 4.

W0 96t0030ti , , ~ , ~ ~ t.~ P ~ PCT/U595I04199 ~ ~

Table 4. Examples of Hydrogen-Consuming Sulfate-Reducing Bacteria' Selected other Optimum Optimum electron donors(D)pH range, temp., Growth Genus/species and acceptors(A)units C requirements S Desuifobact~r genus Acetate(D) 6.0-7.0 20-33 Vitamins,Salts'' cuwatus (DSM 3379) Acetate(D) 6.8-7.2 hvrosenophilus {DSM 6.6-7.0 3380) 14 Desulfobacterium genus 6.6-7.6 20-30 or anilini (DSM 4660}

autotrophicum (DSM 3382) Acetate(D}, formate(D)6.7 25-28 Vitamins", salts' catechoiicum (DSlli 3882) Acetate{D), formate{D)6.9-7.1 28 Vitamins, Nitrate'{A) dithionite 15 ma tii (DSM 4194) Desutfobulb~s genus Propionate(D}, 6.6-7.5 25-40 Vitamin',acetate ethanol(Dj, Nitrate'(A) as carbon eloneatus (DSM 2908) pro~ionicus (DSM 2032) 20 Desulfacoccus genus 6.6-7.6 28-35 Salts',~7tamins niacini (DSM 2650) Formate(D), ethanol{D) Desulfomicrobium genus 6.6-7.5 25-40 Acetate as carbon 2 5 aDSheronum (DSA4 5918)Ethanol(D) 25-30 Reduc.agents baculatum Desulfomona~

Qiera (ATCC 29098} Ethanol(D) 6.6-7.5 30-40 Acetate as carbon W0 96f0030R i';. all ~ i '~x. ~ '~ ~ ~ ~ ~ '~' FG"flU895109199 Table 4 Examples of Hydrogen-Consuming Sulfate-Reducing Bacteria' (cont.) Selected other Optimum Optimum electron donors(D)pH range,temp., Growth Genuslspecies and acceptors(A) units C requirements Desulfomonile tierliei (ATCC 49306) Formate(D} 6,8-7.0 37 Vitamins, reduc. agentsb Desulfonema limcola (DS(v12076) Acetate(D) 7.0-7.G 28-32 Salts', sediment Desulfosarcina variabilis (DSM 2060} 7.2-7.6 33 Dari.ness Desulfotomaculum genus 6.6-7.4 Vitamins?

ueathermicum 50-60 Acetate as carbon?

kuznetsovni (DSM 6115}Acetate(D), formats{D)-7.0 60-65 None ni ri is (ATCC 1999$) 50-60 orientis (ATCC 19365) 25-40 ruminis (ATCC 2393) 25-40 thermoaceto-oxidans Acetate(D), formate(D)6.5 55-6fl Vitamins (DSM 5813) Desulfovibrio genus Ethanol(D) 6.6-7.5 25-40 Biotin africanus (DSM 2603) 34-37 carbinolicus (DSM 3852) desulfuricans (ATCC Nitrate{A), 34-37 Acetate as 2774) nitrite{A) carbon fructosovorans Formats {D} -7.0 -35 Acetate as (ATTC 49200) carbon furfuralis Nitrate(A) -6.8 -38 .

gisanteus Formater{D) 7.5 35 Acetate as carbon;
saltR

aiaas (ATCC 19364) Nitrite(A) 34-37 W~ 96100308 , , ~ ~ ~ ~ ~ ~ ~ PCTIUS95I09199 Table 4. Examples of Hydrogen-Consuming Sulfate-Reducing Bacteria' (cont.}
Selected other Optimum Optimum electron donors(D}pH range, temp., Growth Genus/species and acceptors(A)units C req uirements salexine~ ns (ATCC Formate(D), 34-37 Saltk 14822) ethanol{D) simplex (DSM 4141) Formate(D), -7 -37 Acetate as ethanol(D), carbon Nitrate(A) Ni*z, WO;
z sulfodismu_tans (ATCC 43913) termitidis (DSM >308) v_ulearis (.ATCC 29579) 34-37 Acetate as carbon Thermodesulfo-bacterium 6.6-7.5 commune (ATCC 33708) ~7.0 70 Acetate as carbon mobile (DSM 1276} Formate(D} 70 ' Sources' Widdei and pfennig, 1984; Holt et aL, 1994 " Vitamins are nicotinamide, 1-4-napthoquinone and thiamine in a defined mineral medium.
Reductants are 1 mW NazB~9HzO or 0.:5m144 NazS=O,, ' Denitrified to ammonium.
Vitamins are biotin and p-aminobenzoate ° Salts are 20811 NaCI and 3g/I MgCI= 6F1=O.
'Completely oxidized.
° Salt is 2 to 25 g/l NaCI.
" Salts are >7g/I NaCI and =~ 1 gll MgCI_~6H,0 ' Vitamin is p-aminobenzoic acid.
2 0 ' Salts are > 1 Sg/( NaCI and >2gll MgCI= 6HzO.
'' Salt is 20glI NaCI.
°' Salts are 7-20g~1 NaCI and l-3gJ1 MgClz~6Hz0.
" Vitamins are thiamine, biotin and p-aminobenzoic acid.
° Vitamins are biotin, p-aminobenzoate and nicotinate.
As noted, some of the sulfate-reducing bacteria listed in Table 4 are also nitrate-reducing bacteria because they can also reduce nitrate to produce ammonium. In a preferred, embodimentrt leaching solution is produced in an anaerobic reactor by culturing in the reactor sulfate-reducing bacteria capable of using forntate or acetate, as well as hydrogen as electron donors, and both sulfate and nitrate as electron acceptors. Since anions, such as sulfate and WO 96/00308 * ~~ ~,j ~ <,~ L f ~ ~ ~ t~ ~' PCTlUS95l09199 ,:
nitrate, are reduced, such bacteria are oxyaruon-reducing bacteria. Examples of such bacteria include mesophihc, fresh-water species such as Desrrlfolxxcterinm catechodicarm DSM 3882 (acetate and formate) and Desulfovi6rio simplex DSM 4141 (formate);
mesophilic, salt-water species, such as Desxrlfovi8rio salextgems DSM 2638 (formate); and thermophilic, fresh-water species such as Destrtfomaculum krratetsovii DSM 61 I5 or VKM B-1805 (acetate and formate). Microorganisms with ATCC accession numbers can be obtained from the American Type Culture Collection. 12301 Parklaevn Drive, Rockville, Maryland 20852-1776, tel i-800-638-b597, fax 1-301-231-5826. Microorganisms with DSM accession numbers can be obtained from the Deutsche Sammlung van Mikroorganismen and Zellkulturen GmbH, Mascheroder Weg ib, D-38124 Braunschweig, Germany, tel O11-49 (0)531-2616-336, fax 01 I-49 (0)531-2616-418. niicroarganisms with VICIvt accession numbers can be obtained from the Institute of Biochemistry and Physiology of Microorganisms of the Russian Academy of Science, Pushchino-na-Oke, I42292 Moscow Region, Russian Federation.
In an alternative embodiment, preferred especially for laboratory (process optimization) studies, additional hydrogen consumption is accomplished by purging the headspace of bisulfide leaching reactor 22 through a H=S-scrubbing means (e.g., a zinc acetate solution "bubbler") into a nitrate-fed reactor containing a culture of sulfate-reducing bacteria that are also capable of nitrate reduction, operated in parallel (side-stream) or in series with reactor 22. In this way, "hydrogen-scrubbed" headspace gas is recycled back to reactor 22.
Alternatively, a zinc acetate 2 0 bubbler would not be required if the H,S concentration in reactor 22 were controlled independently by a side-stream bubbler controlled by a sulfide ion-selective electrode that would turn an a H,S-scrubbing bubbler loop when a high H,S setpoint was reached.
In an alternative embodiment, the HtSrst concentration and the HS' concentration may be increased to an appropriate level, and the H= fugacity may be reduced to an appropriate level 2 5 in the environment provided by reactor 22 by contacting the contents of reactor 22 with a stream of gas having an appropriate H=S fugacity and effectively no H,. This stream of gas may be produced biologically by a culture comprising sulfate-reducing bacteria or it may be produced abiotically using conventional means. An equilibrium will be reached that partitions the constituents of reactor 22 of limited solubility between the gas and liquid phases in reactor 3 0 22. Henry's Law can be used to predict equilibrium and steady state constituent Levels.
While gold, silver and platinum-group elements are soluble in bisulfide solutions at ambient (atmospheric) pressures and at room temperature, their solubilities generally increase with pressure and temperature (Krauskopf, K.B., Economic Geolo$v, 46, 858-870, 1951;

WO 9fi/00308 ' ~' ': ; ~' ~.' ~ ~ ~ ~ ~ ~ ~ PCTlU595109199 Weissberg, B.C., Economic Geolos.Tv, 65, 551-556, 1970). For ihis reason, in an alternative embodiment, sulfate-reduction reactor 8 is operated in the thermophilic (50-100°C) and barophilic (over one atmosphere) ranges (e.g., in a submerged, covered heap).
If sulfate-reduction reactor 8 is operated at steady-state at relatively high total dissolved sulfide (H2S ~,q~ + HS- + S-Z ) concentrations (say over about 1,000 mgll in the liquid), then sulfate-reducing bacteria will be enriched in reactor 8 that are relaiively resistant to growth rate inhibition by such total sulfide concentrations. Many investigators have reported that common sulfate-reducing bacteria can grow in media containing over 2,700 mgJl of total sulfides (See Miller, L.P. ( 1950). Formation of metal sulfides through the activities of sulfate-reducing l0 bacteria. Contributions from Boyce Thompson Institute. 16,. 85-89; Saleh, A.M., MacPherson, R., & Miller, J.D.A. (1964). The erect of inhibitors an sulphate reducing bacteria: a compilation, Journal of~olied Bacteriolo~,y ~7 281-293) In a preferred embodiment, sulfate-reduction reactor 8 is operated at a relatively low fatal sulfide concentration (say less than about 1,000 mgll in the liquid) in order to minimize inhibition of the sulfate-reducing bacteria growing in it. This may be achieved by using a vacuum pump or purging gas stream to transfer H=S gas from the headspace of reactor 8 to the liquid in leaching reactor 22.
In one embodiment, a HZS gas pump is used to increase the H:S partial pressure in the transferred gas stream. R'ith this embodiment, the 1-i:S gas removed from reactor 8 is absorbed in a basic (pH >7) solution as dissolved HS ions during the intake portion ofthe pumping cycle.
During a subsequent discharge portion of the pumping cycle, the solution containing dissolved HS' ions is acidified to convert the dissolved HS to H=S gas and the H=S is pumped into leaching reactor 22. In one embodiment, waste sulfuric acid produced by oxidation of metal sulfides is used to acidify the solution containing the dissolved HS- ions.
In an alternative embodiment, H:S gas pumping is accomplished by dissolving it in a liquid solution at a relatively low temperature (e.g., 10°C). The H:S
is then driven out of the solution by heating the liquid to a relatively higher temperature (e.g., 60°C). This form of HAS
pumping is made possible by the significant change in Henry's law coelTtcient for H2S gas with temperature.
Precious metals recovery options include adsorption on activated carbon;
adsorption on ion-exchange resin; and modification of the solution pH, hydrogen fiagacity, or oxidation-reduction potential (ORP). In an alternative embodiment, precious metals are adsorbed on the cell walls of bacteria and the bacteria are separated from the liquid in which they are suspended WO96100308 ,., l: ~ ~ ~ ~ ~ ~ ~ PLTlUS9'Sf09199 :..- f by settling andlor filtration of the liquid after sextling of the ore particles. Options that do not otherwise modify lixiviant solution chemistry are preferable. For this reason, in preferred embodiments, at least reactors 8 and 22, and preferably also reactor 26, are operated together as a single, essentially completely-mixed reactor.
In an alternative embodiment, pregnant solution 24 is degassed to reduce its total dissolved sulfide concentration before andlor concurrent with contacting it with granular activated carbon in precious metals recovery reactor 26. Degassing may be accomplished by pumping gas from the headspace of reactor 26 into the liquid in leaching reactor 22. Precious metals that have absorbed to the activated carbon are eluted into a concentrated solution that is a solvent for the precious metals. Precious metals are recovered from the concentrated solution by conventional means.
Recovered precious metals are converted into products. This may include the operations of separating, smelting and casting of each precious metal into bars or bullion.
Reference is now made to Fig. Z which is a schematic diagram illustrating a second alternative representative embodiment of the invention, with dashed lines representing possible variations in the process and apparatus. In this embodiment, ore 30 preferably undergoes crushing 32 to facilitate exposure of precious metal values in the ore to processing solutions.
Crushed ore 34 then undergoes acid leaching 36 in aerobic reactor 37. If necessary, air 38 containing oxygen and carbon dioxide is added in the acid leaching step Acid-leach solution 40 2 ~ is recirculated through the ore undergoing acid leaching by means of pump 42.
Acid-leached ore 44 then undergoes bisulfide leaching 4G in essentially completely-mixed, anaerobic reactor 47. Bisulfide lixiviant 48 is recircufated through the ore undergoing bisulfide leaching by means of pump 50. The pH of bisulfide fixiviant 48 is established at an optimum pH by pH controller 60 which controls the rate of addition of acid-leached ore 44 and 2 5 acid-leach solution 40 to reactor 47 by means of valves 62 and 64. The sulfate andlor the sulfide concentration in bisulfide lixiviant recirculation loop 76 is monitored by sensorlcontroller 82, which may comprise an ion-specific electrode. Sensorlcontroller 82 is programmed to add up to a stoichiometric amount of electron donor 84, which is a sulfate-reducing bacteria growth substrate such as formate, acetate or methanol, to bisulfide lixiviant recirculation loop 76.
3t~ Pregnant bisulfide lixiviant 66, which contains precious metal values is subjected to gold and silver recovery 68. Recovered gold and silver is converted into products (e.g., bars of essentially pure metal). Spent lixiviant 70 is returned to bisulfide lixiviant recirculation loop 76.

In a preferred embodiment, gold and silver recovery 68 is accomplished by passing pregnant bisulfide lixiviant 66 through activated carbon coh~mn 78.
Leached ore 80 undergoes dewatering 90 by conventional means, such as settling and/or vacuum filtration. Contained bisulfrde lixiviant 92 is retetrned to bisul8de lixiviant recirculation loop 76. Waste ore 94 is disposed ofby using conventional means.
In an alternative embodiment, acid-leach solution portion 96 undergoes base metal removal 98 in base metal removal reactor 100. Excess hydrogen sulfide gas 110 removed from anaerobic reactor 47 is introduced to base metal removal reactor 100 to precipitate iron and other base metals 104. Acid-leach solution portion 102 having a reduced base metal content may be returned to reactor 37, or optionally, to reactor 47.
In an alternative embodiment, excess hydrogen sulfide gas portion 112 undergoes sulfur recovery I 14 in sulfur recovery reactor 116. Recovery of element sulfur I20 may be accomplished by the conventional Claus process or by means ofthe process disclosed in U.S.
Patent No. 4,666,852.
Reference is now made to Fig. 3 which is a schematic diagram illustrating a third alternative representative embodiment of the invention, with dashed lines representing possible variations in the process and apparatus. In this embodiment, sequential processing of heaps 200 and 202 of crushed ore 204 and 205 is accomplished. In heap 200, conventional bio-oxidation of crushed ore particles 200 is accomplished to free precious metals dispersed or occluded 2 0 within the ore. Air 206 may be introduced to heap 200 via plenum 208.
Acidic, base-metal sulfate leach solution 2I0 is collected from the bottom of heap 200 through plenum 208 by means of pump 212. Portion 214 of leach solution is recirculated by means of pump 212 and distributor 2I6 to the top of heap 200.
As was noted above, bio-oxidation of heap 200 may include ore crushing, acid 2 5 . pretreatment, inoculation with appropriate sul$de-oxidizing bacteria, addition of nutrients, recircuIating the biolixiviant and cooling the heap (for 3 to 8 days), and allowing the heap to "rest" (for 3 to 8 days). Additional process steps may include washing heap 200 for an extended period (e.g., 14 days) to remove residual acidity or iron content, and breaking heap 200 apart in order to agglomerate ore 202 with cement and/or lime to make a new heap, such as heap 202.
3 0 Portion 220 of aadic, base-metal sulfate leach solution 210 produced by the bio-oxidation step is introduced to anaerobic, sulfate-reduction reactor 230. In this embodiment of the process, reactor 230 is a side-stream reactor. The rate of addition of portion 220 to reactor ' 230 may be controlled by pH controller 232 which operates valve 234 to create an optimum pH

wo 9GIOO308 ~. '~ .,.; ( ~ C~ ~ 3 ~ ~ PC1YU8~5109199 for precious metals leaching in bisulfide leach solution 238 produced by reactor 230. Preferably, non-toxic electron donor 240 (such as formate, acetic acid (e.g., vinegar), acetate, or methanol--which does not bind effectively to activated carbon), is added to anaerobic reactor 230 to enrich within reactor 230 a microbial culture comprising sulfate-reducing bacteria.
Arraerabic reactor 230 is preferably operated in a pH-stet mode by adding a sufficient pardon 220 of acidic sulfate solution to maintain a neutral pH in reactor 230. In some embodiments, the concentration of dissolved sulfide (HxS, HSw, and S') in the anaerobic reactor is maintained below about 2,500 mg/l to prevent inhibition of the microbial culture comprising sulfate-reducing bacteria.
In a preferred embodiment, base metals 244 (such as iron) are precipitated in downstream settling tank 250, and portion 252 of clarified bisulfide lixic~iant 254 is recirculated to reactor 230. The rate of recirculation of portion 252 is preferably chosen so that reactor 230 and settling tank 250 are operated together as a single, essentially completely-mixed reactor.
Headspace 260 of reactor 230 and headspace 262 of settling tank 250 are preferably connected by~ conduit 264. Excess hydrogen sulfide gas (H~S) 266 produced in anaerobic reactor 230 (e.g., that amount over about Z. X00 mg9) and tank 250 is preferably removed.
In some embodiments, excess hydrogen sulfide gas undergoes sulfur recovery 270 to produce elemental sulfitr 273. By means afreaMOr 230 and tank Z50, sulfate-reducing bacteria are used to create clarified, approximately neutral (pH 6 to 9) leaching solution 254 comprising bisulfide tans and a low concentration of dissolved and suspended base metals. Bisulfide lixiviant 254 and 2 0 headspace 260 comprise the reactor envirorunent of reactor 230.
In another preferred embodiment, excess H=S gas produced in reactor 230 is removed from headspace 260 andfor headspace 262 by means of a H=S gas pump (not shown) and transferred into clarified bisulfide iixiviant 254 downstream from settling tank 250. In this was the concentrations of H,S~,~~ and HS in the lixiviant are increased after mast of base metals 244 are removed from it.
In a preferred embodiment, heap 200 is undergoing bio-oxidation while a second heap 202, which has previously undergone bio-oxidation, undergoes leaching with bisulfide lixiviant.
In a second process step, oxidised ore 205 is preferably covered with cover 208 and submerged in bisulfide iixiviant 282 to exclude oxygen. Heap 202 is leached by recirculating portion 292 of 3 0 neuttal bisulfide lixiviant 282 saturated with H=S through it by means of plenum 284, pump 286, and distributor 290. In an alternative embodiment, the H=S pattial pressure is increased by introducing the lixiviant [andlor HZS gas having a low concentration (less than 1,000 parts per million by volume) of H; gas] under pressure at the bottom of a heap via plenum 284 which is Vf~O 96100308 y , .- ~ f ~ ~ ~ ~ y PCTlUS95109199 submerged in lixiviant 282, causing H5- ion concentrations to increase in direct proportion to the increase in H2S partial pressure. This may increase the concentration of dissolved sulfide (H2S, HS-, and S =) in heap 202 above ?, 500 mg/l. In a preferred embodiment, anaerobic reactor 230, settling tank 250, and heap 202 are operated together as a single, essentially completely-mixed reactor by recirculating portion 294, from heap 202, to reactor 230.
In an alternative embodiment, pressure sensors are placed at multiple paints throughout the system far safety reasons. This provides a warning system for users of the system, since releases of H,S~g~ can he toxic. Low pressures sound an alarm, indicating a leak somewhere, while high pressures indicate unsafe operation. The use of multiple gauges pinpoints the source 1 G of the problem quickly. The pressure gauges are also used to monitor and regulate the HzS,&~
pressures to optimize the solubility of the gold and silver.
Additionally, conductivity and total dissolved solids meters are placed in the effluent streams of the sulfate-reducing reactor in order to measure the ionic strensnh of the solvent.
The meters are used to monitor the ionic strength of the solvent, which controls the activity coefficients of the gold and silver complexes, HZS~,q~, HS-, and H,~r,.
Control of the activities of these compounds increases the efficiency of solubilizing the Bald and silver.
Complexed gold and silver in pregnant portion 300 of lixiviant 282 is recovered continuously from the lixiviant solution in reactor 302. Recovery may be accomplished in a conventional manner by adsorption on activated carbon or by precipitation on zinc dust or by modifying either the solution pH, hydrogen fugacity, or oxidation-reduction potential (ORP).
R4etal that has been recovered from activated carbon eluent by electrowinning or zinc dust may be smelted to recover precious metal values as products such as jewelry or electronic system components. Barren lixiviant solution 306 is recycled to heap 202.
Working Example No. 1 A chemostat having a working (liquid) volume of 5 liters and a headspace volume of 2.5 liters was operated at a dilution rte of O.OOfi per hour for over 6 hydraulic detention times sa that steady state conditions were achieved. A sulfate-reducing bacteria growth medium comprising farmate ions was pumped into the chemostat at a constant rate. The pH of the liquid in the chemostat was maintained at 7.0 by~ means of a pH controller that added bio-oxidation process efr7uent (acidic metal sulfate solution) to the reactor as required.
Sufficient fomtate ions and sulfate ions were introduced to the chemostat to produce a headspace H=S partial pressure of about 1 atmosphere. Achievement of this partial pressure of W096J00308 :w r . .~ rz 's ~,, ~ PCTIUS95J09199 HzS was assured by purging the chemostat wish a gas contaitung 99.5+ percent H=S at the beginning of the experiment.
The concentration of H= gas in the chemostat headspace before it was purged and in the gas used to purge the chemostat was measured by means of a gas chromatograph with a thermal canductir~ity detector. The concentration of H~ in the headspace was about 300 parts per tttilIion (ppm) by volume and the H: concentration in the purging gas was about 200 ppm.
The liquid level and the liquid volume in the chemostat was kept constant by withdrawing liquid and headspace gas from the chemostat at a greater rate than liquid was added to the chemostat. The chemostat effluent contained about 200 mg/1 of formate. The effluent was discharged to a reservoir, the headspace of which was connected to the headspace of the chemostat.
A square of gold foil about 0.1 inch on a side and 0.025 inch thick was placed in a 160-milliliter (ml) serum bottle and a Teflon ~ septum stopper was crimped on the bottle mouth.
The bottle was purged with oxygen-free nitrogen gas and 100 ml of chemostat effluent was I S transferred to the battle without exposing it to air.
The contents of the bottle were then purged three times with the afore described HjS gas mixture at about 3-day intervals. Within four hours of the itutial purging, the liquid in the bottle took on a bright yellow color. Testing of a six-ml sample of the liquid plus two ml of aqua regia revealed that the liquid contained about 0.3 mgll of gold.
2 0 A 10-ml sample of the liquid was withdrawn from the bottle and introduced anaerobically to a similar serum bottle containing washed granular activated carbon and the Ii:S
gas mixture. The liquid in contact with the activated carbon immediately became colorless indicating adsorption of the gold on the activated carbon granules. Assays of the activated carbon revealed that gold was adsorbed on the carbon.
Working Example No. 2 Reference is now made to Fig. 4 which is a schematic diagram illustrating a fourth alternative representative embodiment of the invention, with dashed lines representing possible variations in the process and apparatus. In this embodiment, an experiment was conducted to 3 0 illustrate the disclosed method and apparatus on low-grade samples of gold ore. Experimental procedures and results are presented below.
Each I .5 kilogram ore sample was ground to a fine powder that could pass through a 150-mesh sieve (sieve opening = 0.004 inch). The entire sample was split into three W~09G100308 , ~; 2 ~ ~ j~ ~ q ~ PC'TlUS95109199 representative samples of approximately 500 grams each. The first representative aliquot of the ore was assayed three times for gold and silver content, as well as for the presence of trace elements. The second and third representative aliquots were bio-oxidized to oxidize (and solubiIize} metal sulfides and mobilize gold and silver values.
Bio-oxidation was accomplished in aerobic, stirred, batch reactor 312 having a working volume of 5 liters. Batch reactor 312 was placed in a water bath (not shown) having a temperature of 35°C. About 1,000 grams of the ground ore was suspended in about 5 liters of an acidic :rhiohacillusferrnoxidarzs medium in the reactor. The acidic medium as described in ASTM Standard E 1357 contained the constituents shown in Table 5 and its pH
was adjusted to pH 2 with concentrated sodium hydroxide (NaOH). Air and carbon dioxide were introduced into the suspension by pumping air into it at a relatively high rate with air pump 314. The suspension was inoculated with an active culture of T?riohacilhrs ferrooxidafrs, ATCC 13661, obtained from the American Type Culture Collection at the address given above.
The progress of bio-oxidation was monitored by measuring pH (with first pH monitor 318) and dissolved iron concentrations in the acidic medium.
Table ~. Bio-Oxidation Medium Compound Concentration, mg/I

Ammonium sulfate, (NH,)xSO, 300 Calcium nitrate,Ca(N0,)z 1 Magnesium sulfate, MgSO,7H=0 50 Potassium chloride, KCI 10 Potassium phosphate dibasic, K:HPO, 50 Iron sulfate, FeSO,7H,0 44.22 Sulfuric acid, H,SO,195-98%) 2.6 ml After a period of ten to twelve days, when the rate of increase in iron concentrations in the acidic medium slowed and the pH stabili~aed, ore 322 was separated from liquid supernatant 3 0 320 by settling and drying in an oven at 140°C. A first representative portion of dried, bio-oxidized ore 324 was subjected to conventional cyanide e~,~traction, and then assayed For gold content to provide a basis of comparison with the bisulfide extraction.

9 ~ ~ ~ PC'flUS95/09199 WO 9Gl00308 ~~
i 5 F. ~ ~
A leaching solution comprising dissolved hydrogen sulfide gas and bisulfide ions was produced in continuously stirred tank reactor (CSTR or chemostat} 32b having a working (liquid) volume of five liters and a headspace volume of 2.5 liters. Chemostat 326 was placed in a water bath (not shown) having a temperature of 35°C. Chemostat 326 was started in a batch mode by placing a sulfate-reducing bacteria medium in the chemostat, inoculating the chemostat with wild sulfate-reducing bacteria and allowing the culture to acclimate for 5-7 days.
After an acclimation period, sulfate-reduction medium 328 containing the constituents shown in Table 6 and formate as a carbon source was pumped into reactor 326 by pump 330 at a rate that produced a dilution rate of about 0,005 per hour. Liquid effluent was removed from the chemostat by pump 332 at the rate required to maintain the liquid level in the chemostat at a set level and discharged to efiiuent storage container 342. This dilution rate produced a mean cell residence time in the reactor that was much less than the maximum specific growrth rate of the sulfate-reducing bacteria used to inoculate it. Chemostat 326 was operated in a pH-stat mode at pH 7.0 by continuously monitoring the pH of the liquid in chemostat 326 with pH
monitoricontroller 336, and by intermittently pumping acidic supernatant 338 produced by the bio-oxidation step into chemostat 326 with pump 340. Addition of acidic supernatant 338 to chemostat 326 increased the dilution rate to about 0.006 per hour. The chemostat headspace was periodically purged with hydrogen sulfide from canister 334 to maintain positive pressure within the reactor. After chemastat 326 had been operating for about three hydraulic detention times and had reached steady state, the effluent from the reactor was used as solvent in leaching experiments Pyrex serum bottles 348 with a capacity of 160 ml were used as batch leaching reactors.
The reactors had previously been washed with aqua regia because Pyrex is known to adsorb gold complexes under certain conditions. Representative four-gram portions of the bio-oxidized 2 5 ore were added to the reactors. The reactors were augmented with 4 gram portions of prewashed 4-12 mesh activated carbon. Effluent collected from the chemostat was dispensed in I00 m1 aliquots into the leashing reactors by pump 346. The reactors were immediately capped, sealed, purged and pressurized to 1 atmosphere absolute with 99.5 percent pure hydrogen sulfide gas. The reactors were then placed in both 35 and 65 °C
incubators. The experiments were mixed by hand about two times daily and purged and pressurized with hydrogen sulfide gas at least every 48 hours.

WO96l00308 " ' ~ , PCT/US95/09199 Table 6. Sulfate-Reduction Medium Compound Concentration Solution 1: Mineral Media mg/l Ammonium chloride, NH,CI 300 Calcium chloride, CaCI: H,O 150 Magnesium chloride, MgCl2 6H=0 400 Potassium chloride, KCI 500 Potassium phosphate monobasic, ICH_PO~ 200 Sodium chloride, NeCI 1,200 Yeast extract 6.5 Solution 2: Trace Minerals' mgR

Boric acid, H,BO, 60 Cobalt chloride, CoCI. 6H=O 120 Copper chloride, CuCI= GH_O 15 Ferric chloride, FeCI4H,0 1500 2 0 Manganese chloride, MnCl2 4H~0 100 Nickel chloride, NiCI6HZ0 25 Sodium molybdate, NaMoO; 2H_O 25 Zinc chloride, ZnCI 70 Hydrochloric acid, HCI (25l0) 6.25 ml ?5 Solution 3: Vitaminsb mg/

Biotin 50 Calcium pantothenate 125 Thiamine 250 30 p-Aminobenzoic acid 250 Nicotinic acrid 500 Pyridoxamine 1,000 lotion 4: Sodium Bicarbonate Solution' mg/100 ml 35 Sodium bicarbonate, NaHCO, 8,500 Distilled water 100 ml Solution 5: Selenium TunEstate Solutian° mgt!
Sodium hydroxide. NaOH 500 ~i 5 WO l6l00308 ~ ~ g C~ ~ jt1 ~ PCTtUS95109I49 ,. x .,. ,~, ; ~ f '~
,~W y. ,;. cW s Table 6. Sulfate-Reduction Medium (coot.) Compound Concentration Sodium selenite, Na=SeO; SH;O 12 Sodium tungstate, Na=WO; 2H:0 16 Solution 6: Vitamin BI2' mg,'200 ml Cyanocobalamia 20 Distilled water 200 ml Add 1 ml per liter of Mineral Media " Add 200 pl per liter of Mineral Media ' Add 30 ml per liter of Mineral Media ~ Add 250 pl per titer of &Iineral Media 'Add 200 lCl per liter of Mineral Media Samples of the liquid phase from the reactors were taken several times throughout the experiment. The 6 ml liquid samples were taken from well settled reactors and filtered with a 0.2 pm millipore filter into IO ml serum bottles. The samples were preserved in 2 ml aqua regia and analyzed for gold concentrations by inductively-coupled plasma atomic emission spectroscopy (ICP) analysis.
Upon completion of the experiments, all the components of the experiments were aaalyzed for gold and silver concentrations. Liquid samples were taken from the reactors, then 2 0 the bottles were opened and the components were separated. The reactors were shaken and poured through a small mesh sieve (to collect the activated carbon) into a vacuum filter funnel containing a Whatman glass fiber filter. The liquid passed through and the ore portion was collected on the filter. The activated carbon was washed with distilled water, blotted dry with filter paper and dispensed into serum bottles. The ore and the filter were removed from the 2 5 apparatus and inserted into serum bottles. Both the carbon and the ore samples, were analyzed by fire assay for gold and silver concentrations. The liquid samples were prepared as described above and analyzed for gold by ICP analysis.
Bisulfide leaching results are presented in Table 7. The relatively low recovery percentages were attributed to infrequent mixing of serum bottle contents and infrequent 3 0 purging with HxS gas. Because the rates of metal dissolution reactions are controlled by the rates at which reactants can reach and products can leave the metal surface as well as by reactant and product concentrations at the metal surface, subsequent experimental designs addressed these factors.

W096100308 ' ' ' ) ~ PCT/U595109199 ~ - l; ~t~.S~~
Table 7. Bisulfide Leaching Results Placer PlacerPlacer Placer Mine Suurcc BatrickBarrickBarnckBarrickdrnne dome dome dcmte Leaching temp. 35 36 65 66 35 35 65 65 ~~

Solventtrcatmeat non- non- non- non-(tiltratiott) filteredtilten-~1filteredfilteredIileemdfilteredfilteredfiltered Gold wncentrations Bio-oxidizedorc 0.10 0.105 U.10~ 0.10 O.Ui2 0.052 0.052 0.052 (ozhon) Bisultide LeachedO.t)82O.U72 0,076 0.078 0.039 U.035 O.U36 U.037 ore (oz/ton) Crold recoven~ 22 31 28 25 25 34 32 U

percent INDUSTRIAL APPLICABILITY
The invention has utility as a means of extracting precious metals from ore that is being mined in situ or ex situ. The invention can also be used to recover precious metals from scrap.
Many variations in configurations have been discussed and others will occur to those skilled in the art. Some variations within the scope ofthe claims include biotic and abiotic means far producing the bisuifide lixiviant and other 1-I=S gas pumping schemes. All such variations within the scope of the claims are intended to be within the scope and spirit of the present invention

Claims (20)

THE EMBODIMENTS OF THE INVENTION FOR WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process of making a product, said product comprising a precious metal that can form a complex comprising a sulfide, said complex having an aqueous solubility, said process comprising the steps of:
introducing an ore of said precious metal into an anaerobic reactor environment, said reactor environment having an activity of dissolved hydrogen sulfide gas, an activity of dissolved sulfide ions, and a fugacity of hydrogen gas;
increasing said fugacity of hydrogen sulfide gas and said activity of sulfide ions and reducing said fugacity of hydrogen gas, whereby the solubility of the precious metal-sulfide complex is increased;
removing the precious metal-sulfide complex from said reactor environment;
separating said precious metal from said metal-sulfide complex; and forming said product from said precious metal;
wherein said increasing step and said reducing step are accomplished by at least one step selected from the group of:
contacting said reactor environment with a culture of an oxyanion-reducing bacterium, and contacting said reactor environment with a stream of gas comprising hydrogen sulfide gas and less than 1,000 parts per million by volume of hydrogen gas.

2. The process of claim 1 wherein said dissolved sulfide ions comprise dissolved bisulfide ions.

3. The process of claim 1 wherein said oxyanion-reducing bacterium is a sulfate-reducing bacterium.

4. A method for extracting a precious metal from an ore comprising the steps of producing a leaching solution, said solution comprising dissolved hydrogen sulfide gas and bisulfide ions, and having a fugacity of hydrogen gas;
reducing the fugacity of hydrogen gas in said leaching solution by means of an oxyanion-reducing bacterium;
exposing said ore to said leaching solution; and recovering said precious metal from said leaching solution.

5. The method of claim 4 wherein the producing step is carried out by means of a sulfate-reducing bacterium.

6. The method of claim 4 wherein the oxyanion-reducing bacterium is selected from the group of:
an acetogen, a methanogen, a sulfate-reducing bacterium, and a nitrate-reducing bacterium.

7. The method of claim 4 wherein the oxyanion-reducing bacterium is selected from the group of:
Desulfobacterium catecholicum (DSM 3882), Desulfovibrio simplex (DSM 4141), Desulfovibrio salexigens (DSM 2638), and Desulfomaculum kuznetsovii (DSM 6115 or VKM B-1805).

8. A process for precious metal leaching comprising the steps of biologically reducing a dissolved sulfate under anaerobic conditions to produce a bisulfide lixiviant having an approximately neutral pH and a hydrogen gas fugacity of less than 0.001 atmospheres, leaching a first portion of an ore comprising at least one precious metal by exposing said ore to said bisulfide lixiviant, and recovering said at least one precious metal from said bisulfide lixiviant.

9. The process of claim 8 further comprising biologically oxidizing said first portion of the ore prior to said leaching step.

10. The process of claim 8 wherein said dissolved sulfate is produced by biologically oxidizing a second portion of the ore.

11. The process of claim 8 wherein said biologically reducing step and said leaching step are accomplished in an essentially completely mixed reactor and at a pH in the range of 6 to 8.

12. The process of claim 10 wherein said biologically oxidizing step occurs in a first heap of the ore and said leaching step occurs in a second heap of the ore.

13. The process of claim 12 wherein said recovering step is comprised of exposing pregnant bisulfide solution to activated carbon and eluting said precious metals from said activated carbon.

14. An apparatus for extracting a precious metal from an ore comprising:
means for producing a leaching solution, said solution comprising dissolved hydrogen sulfide gas and bisulfide ions, and having a fugacity of hydrogen gas and for reducing the fugacity of hydrogen gas in said leaching means for exposing said ore to said leaching solution; and means for recovering said precious metal from said leaching solution.

15. The apparatus of claim 14 wherein the means for producing and for reducing comprises a sulfate-reducing bacterium.

16. The apparatus of claim 14 wherein the oxyanion-reducing bacterium is selected from the group of:
an acetogen, a methanogen, a sulfate-reducing bacterium, and a nitrate-reducing bacterium.

17. The apparatus of claim 14 wherein the oxyanion-reducing bacterium is selected from the group of:
Desulfobacterium catecholicum (DSM 3882), Desulfovibrio simplex (DSM 4141), Desulfovibrio salexigens (DSM 2638), and Desulfomaculum kuznetsovii (DSM 6115 or VKM B-1805).

18. An apparatus for precious metal leaching comprising:
means for biologically reducing a dissolved sulfate under conditions that produce a bisulfide lixiviant having a hydrogen gas fugacity of less than 0.001 atmospheres, whereas the conditions that produce a bisulfide lixiviant comprise absence of dissolved molecular oxygen, presence of an electron donor, presence of a source of carbon, and pH in the range of 6 to 9;
means for leaching a first portion of an ore comprising at least one precious metal by exposing said ore to said bisulfide lixiviant; and means for recovering said at least one precious metal from said bisulfide lixiviant wherein the means for recovering said at least one precious metal from said bisulfide lixiviant is selected from the group of:
means for adsorbing said at least one precious metal on activated carbon, means for changing the pH of said bisulfide lixiviant, means for increasing the hydrogen fugacity of said bisulfide lixiviant, means for changing the oxidation-reduction potential of said bisulfide lixiviant, means for reducing the pressure of said bisulfide lixiviant, and means for reducing the temperature of said bisulfide lixiviant.

19. The apparatus of claim 18 further comprising means for biologically oxidizing said first portion of the ore prior to its introduction into said means for leaching.

20. The apparatus of claim 18 wherein said means for biologically reducing a dissolved sulfate comprises a completely mixed stirred tank reactor operated at a dilution rate of about 0.005 per hour and said apparatus further comprising means for monitoring and controlling the ionic strength of the lixiviant and thereby optimizing an activity coefficient of a reactant or a product.